Self-aggression presents a serious problem in laboratory macaques that cannot be socially housed for scientific reasons. We have had only limited success in reducing self-aggression through enriching the environment with puzzle feeders that offer food rewards, rawhide bones, and other manipulable devices. Housing adult males so that they are not directly facing other adult males seems to help in some cases, but does not eliminate self-aggression. Psychotropic drugs such as Valium (a benzodiazepine derivative) and Prolixin (a phenothiazine derivative) offer some help in severe cases where the animals are prone to injure themselves. However, Valium must be administered several times a day, and Prolixin can have some serious side effects, including tardive dyskinesia and long-term behavior changes. The synthetic steroid cyproterone acetate (CA) has been used to treat deviant sexual and aggressive behavior in men. We continue to test the efficacy of CA on reducing self-biting and other abnormal behaviors in singly-housed adult male rhesus macaques. In addition, we are assessing reproductive recovery of these males after prolonged suppression of gonadal hormones. We are measuring serum testosterone levels and sperm quality (count, motility, morphology, percent live) before, during, and after treatment. There is also some evidence that plasma testosterone levels correlate inversely with the robustness of the immune system. Therefore, we are conducting in vitro lymphocyte proliferation assays throughout the study for immunological assessment.